Al.sub.2 O.sub.3 composites are excellent in corrosion resistance, oxidation resistance and wear resistance and they have been known, together with Si.sub.3 N.sub.4 ceramic, etc., as structural materials suitable to machinery parts, jigs, tools, etc., i.e., as engineering ceramics. Although Al.sub.2 O.sub.3 composites are poor in the strength and fracture toughness, as compared with Si.sub.3 N.sub.4 ceramic, since they are sinterable at low temperature and highly pure Al.sub.2 O.sub.3 powder can be available easily at a reduced cost they are outstandingly advantageous in view of the economy or mass production and various studies have been made for improving the foregoing subject in view of the characteristic of Al.sub.2 O.sub.3 composites.
For instance, U.S. Pat. No. 4,554,445 discloses Al.sub.2 O.sub.3 composites in which acicular SiC whiskers are dispersed in an Al.sub.2 O.sub.3 series matrix. The Al.sub.2 O.sub.3 ceramics in which SiC whiskers are dispersed can be produced by dry-mixing a powder of Al.sub.2 O.sub.3 with an average grain size of 0.3 .mu.m and SiC whiskers and then sintering them under uniaxial pressing under the condition of 1850.degree. C., 41 MPa for 45 min and it is described that the fracture toughness is improved by about 8 to 9 kg/mm.sup.3/2 as compared with conventional Al.sub.2 O.sub.3 composites and the fracture strength can also be improved up to 800 MPa (81.6 kg/mm.sup.2).
However, even the Al.sub.2 O.sub.3 composites having SiC whiskers dispersed therein are still poor in view of the strength and the toughness as compared with Si.sub.3 N.sub.4 ceramic. Further, in a case where Al.sub.2 O.sub.3 composites partially contain coarse crystals with the reason as described later, the strength and the toughness are further lowered than those of the conventional Al.sub.2 O.sub.3 composites, and the relatively high strength or toughness as described above can not easily be reproduced.
That is, coarse crystals are grown in the production process as described above, because the sintering temperature has to be increased owing to the addition of SiC, etc. and, for example, the sintering temperature increases as high as 1850.degree. C. Since SiC whiskers have an effect of suppressing crystal grain growth, no abnormal growth in the crystal grains is caused during sintering if the SiC whiskers are uniformly dispersed. Abnormal growth of the crystal grains occurs in a case where SiC whiskers are localized to result in portions lacking in SiC whiskers.
In view of the above, as a method of suppressing the formation of such coarse crystals, it may be considered a method of at first adding SiC whiskers to a solvent to form a slurry-like mixture (hereinafter simple referred to as a slurry) and then mixing the slurry with an Al.sub.2 O.sub.3 powder for uniformly dispersing the SiC whiskers. However, it is extremely difficult to uniformly disperse the SiC whiskers in the solvent and uniformly disperse the SiC whiskers in the solvent and uniform dispersion of the SiC whiskers in the slurry requries long time mixing. On the other hand, since the long time mixing breaks the acicular SiC whiskers to remarkably lower the aspect ratio, the SiC whiskers can no more contribute to the improvement of the strength and the toughness in the above-mentioned method, by which no improvement can be expected for the strength and the toughness of the sintering product. In addition, in the method as disclosed in the U.S. patent, the mixture of the Al.sub.2 O.sub.3 powder and the SiC whiskers are sintered under uniaxial pressing and the molding is applied only by the press-sintering. Accordingly, the density of the ceramics is relatively low in the production method described above and since it is difficult to increase the relative density to greater than 95%, there is also a problem that products of high density and high toughness are difficult to obtain. Further, there is also a problem that it is extremely difficult or substantially impossible to obtain products of large size or complicate shape.
By the way, for the engineering ceramics having high hardness and high strength at high temperature, application uses as throw-away tips (hereinafter simply referred to as tips) used as cutting tools for workpieces of poor machinability have been expected. Spheroidal graphite cast iron or high manganese steel are known as such workpieces of poor machinability, and both of them are materials involving great difficulty in machining because of high strength and poor thermal conductivity in the case of the spheroidal graphite cast iron and because of hardening of the material during cutting in addition to the reason as above in the case of the high manganese steel.
Heretofore, superhard tip of excellent hardness and strength at high temperature have been used for cutting the workpieces of poor machinability. However, even with the method of cutting the workpieces of poor machinability by using such superhard tips (hereinafter referred to as a conventional superhard tipping method), cutting is impossible unless the cutting rate is extremely slow (about less than 30 m/min) thus requiring a long time for machining to greatly worsen the cutting efficiency. In view of the above, Al.sub.2 O.sub.3 --TiC ceramic or Si.sub.3 N.sub.4 ceramic have been developed and put to practical use in recent years as tips capable of cutting the workpieces of poor machinability at a relatively high speed (about 30 to 60 m/min) instead of the superhard tips has described above.
However, even the Al.sub.2 O.sub.3 --TiC ceramic tips, still involve a problem that they have no sufficient toughness and have low chipping-resistance. Accordingly, in a case where they are applied to the cutting of workpieces of poor machinability such as high manganese cast iron, cutting at an extremely small cutting depth (below 1 mm) is necessary and require a long time for the machining and worsen the cutting efficiency. On the other hand, in the case of cutting the workpieces of poor machinability such as spheroidal graphite cast iron by using tips made of Si.sub.3 N.sub.4 ceramic, since tips made of Si.sub.3 N.sub.4 ceramics are readily abraded due to the reaction with cutting powder, there is a drawback that the tip life is extremely short.
As has been described above, the conventional method employed for the machining of the workpieces of poor machinability by using the conventional tip material (hereinafter simply referred to as conventional ceramics tip method) involves the problems that cannot sufficient cutting performance can be obtained.